High-Resolution Na I and Ca II Absorption Observations toward M13, M15, and M33

Abstract

ABSTRACTWe present high resolution (R = 60000) measurements of the Na I D1 & D2 (5890 Å) and Ca II K (3933 Å) interstellar absorption line profiles recorded toward several post-AGB stars located within the M13 and M15 globular clusters, supplemented with a lower resolution spectrum of the Ca II K-line observed in absorption toward an Ofpe/WN9 star in the central region of the M33 galaxy. The normalized interstellar absorption profiles have been fit with cloud component velocities, doppler widths, and column densities in order to investigate the kinematics and physical conditions of the neutral and partially ionized gas observed along each sight line. Our Ca II observations toward M13 have revealed three absorption components that can be identified with galactic Intermediate Velocity Clouds (IVCs) spanning the -50 > Vlsr > -80 km s-1 range. The Na I/Ca II ratio for these IVCs is < 0.2, which characterizes the gas as being warm (T ∼ 103 K) and partially ionized. Similar observations toward two stars within M15 have revealed absorption due to a galactic IVC at Vlsr ∼ +65 km s-1. This IVC requires at least three different cloud components to fit the observed Na I and Ca II profiles, which is consistent with the theoretical scenario in which possible cloud-to-cloud collisions could result in triggering the star formation process within such complexes. Ca II K-line observations of a sight line toward the center of the M33 galaxy have revealed at least six gas cloud components. A cloud at Vlsr ∼ -130 km s-1 is either an IVC associated with the M33 galaxy occurring at +45 km s-1 with respect to the M33 local standard of rest, or it is a newly discovered HVC associated with our own Galaxy. In addition, three clouds have been discovered in the -170 > Vlsr > -205 km s-1 range. Two of these clouds are identified with the disk gas of M33, whereas the cloud at -202 km s-1 could be IVC gas in the surrounding halo of M33.